Q - How much does it matter what I choose at first level?
Some decisions - where are my stat bonuses - am I a spell caster - which is my School of Magic - have long lasting impact on your character. BUT do I take 1 rank in Thrown or 1 rank in Bow does not. Over time this single skill point becomes unimportant, and you should not worry about spending points in a skill that later proves to be of little use to your character. This should free you up to choose more quickly and choose more based upon gut instinct of your character conception rather than as part of some careful balanced plan.
Q - Why do skills cost double every level?
I wanted an exponentially increasing scale, but the arithmetic involved is just confusing (points for the next rank versus total points spent). This scale (1/1/2/4/8) is the only solution where the sum is the next number in the list.
Q - Why the restrictions on how much you can spend in one skill?
It would be boring if you had all your points in one skill, and it would be far too attractive for spell casters, so this forces some variety.
Q - Why the "at least half must be in magic" restriction?
Originally spells cost double, but at higher levels everyone started taking a couple of levels in spells as they are so useful. This didn't make much sense to me, and I tried a solution but when I worked out the possibilities for starting characters I was having to give too many points. I then looked for an alternative simpler solution, and found this worked well. Spell casters can have an interesting set of spell lists or other skills, and people can't just take a couple of levels in spells.
Q - Why 8 skill points at first level?
I originally had 10 points (as per this old post) but some of the rules got tweaked (e.g. points for spells and bonus skills) so I went back to re-examining this number. 10 is an attractive choice as the number of XP for each level is simple, but instead I played around with what options you got with different numbers of points and found that 8 gives you diversity and enables most character concepts but more means means you can start too powerful. With the "no more than half in one skill" rule you get to be rank 1, 2, or 3 which allows you variety but also a lot of growth potential. In the end I chose 8 over 10 because it works better with the bonus skills and I like how you can be rank 3 in two skills.
Q - Why split Magic into these disciplines?
I've used systems where Magic is split along the source of the Magic (e.g. divine versus arcane). I've used systems where Magic is split along the intended purpose of the magic (e.g. spell lists for attacking, or to make walls). I thought about it long and hard and decide I wanted it split along how the magic acted. Initially I started with Air/Earth/Water/Fire which I've always thought of as being like Gas/Solid/Liquid and the Heat which moves matter between these states. I then thought about the other forms of energy instead of Heat so came up with Light/Sound/Electricity, and the anti-Heat spell list Ice (I love the joke of an anti-Microwave oven for cooling things quickly). Then there are the ways in which you can affect matter so I came up with just Force (as Magnetism seems not very useful, and all the other forces can be munged into one). Then there's forms of life you could affect - Plant, Animal, Human. Then there's real world mystical mumbo jumbo such as ESP, fortune telling, and Summoning Demons which meant Humans split into Body/Mind Control and added Divination and Summoning. Then there's fictional mumbo jumbo which gives you anti-Undead and Time & Space. These fit nicely into 4 lists for 4 areas with a flavour for each and when I allocated all the spells I wanted I had a pretty even distribution straight away. The only thing I can see is missing according to this classification is...
Q - Why is there no Necromancy?
Necromancy is magic through the power of dead bodies, OR is controlling the undead for nefarious purposes. This exists in the game, but isn't an option for players in the rules as written. There are, of course, evil NPC Necromancers - and there is no reason why characters shouldn't embark upon finding out the secret of how to become one.